Nuclear magnetic resonance (NMR) can be used to determine properties of a substance. An NMR procedure typically includes applying a static magnetic field to the substance. The static magnetic field generates an initial magnetization of atomic nuclei within the substance. Then, an NMR system is used to apply an oscillating magnetic field at a particular frequency to the substance. The oscillating field is composed of a sequence of pulses that tip the magnetization of the atomic nuclei away from the initial magnetization. The sequence of pulses can be arranged so that pulses and the static field interact with the nuclei to produce a resonant signal composed of “echoes” within at least a portion of the substance. The portion of the substance where the resonant signal is generated is known as a “shell.”
The resonant signal is detected and then used to determine NMR properties such as T1 relaxation time, T2 relaxation time and attenuation of the signal due to molecular diffusion. These NMR properties can be used to determine the properties of the substance within the shell.
The pulse sequence is typically repeated a number of times so that the resonant signal can be more accurately determined. The next pulse sequence is not initiated until the atomic nuclei within the shell reach thermal equilibrium and are aligned with the initial magnetization. In some cases, it may take several seconds (e.g., 10 seconds) for the shell to reach thermal equilibrium. This means that the NMR system sits idle while the shell reaches thermal equilibrium.